The present invention relates to helicopter construction and more particularly to the rotor head mounting assembly thereof. A vertical drive shaft extends upwardly from a helicopter and a rotor hub is affixed to the drive shaft and carries the rotor blades for rotation with the drive shaft. It can easily be appreciated that the connection of the rotor hub to the drive shaft must be such that the hub and the drive shaft move as one and no movement is allowed therebetween to permit vibrations which would lead to ultimate failure of the assembly.
The invention is an improvement on a helicopter rotor head assembly disclosed in U.S. Pat. No. 3,112,941 to Dutton which issued Dec. 3, 1963. The disclosure of the Dutton patent is incorporated herein by reference thereto. The Dutton patent discloses a rotor head assembly, including an elongated, substantially cylindrical rotor drive shaft having an inward taper near the upper end thereof, and having a threaded portion beginning proximate the smallest diameter portion of the taper and extending upwardly the remaining length of the drive shaft. The rotor shaft has a vertically splined portion about the outer surface thereof below the taper and a rotor hub adapted to carry a plurality of helicopter rotor blades is received by the rotor shaft. The rotor hub has a longitudinal bore therethrough with a first tapered portion at the upper end thereof conforming to the taper on the rotor shaft. The bore is internally splined below the first tapered portion to conform to and mate with the splined portion of the rotor shaft for secure rotation therewith. The bore expands radially, outwardly, and downwardly at a location below the internal splines to produce a second tapered portion therewithin, and a mounting nut is screwed downwardly upon the drive shaft threaded portion to apply a force downwardly upon the hub to firmly engage the taper of the rotor shaft with the first tapered portion of the hub. Additionally, a plurality of circumferentially spaced compression bolts mounted on the rotor mounting nut act to further insure firm engagement between the hub and drive shaft.
According to the Dutton patent, a lower annular cone member is bolted to the bottom surface of the hub and is forced upwardly against an annular flange on the rotor drive shaft in order to secure the drive shaft to the rotor hub at the lower end thereof.
The purpose of the somewhat elaborate assembly disclosed by Dutton is to insure a secure, firm engagement between the rotor hub and the rotor drive shaft utilizing a construction which may be assembled and disassembled with the necessity of large hydraulically operated tools. In addition to permitting the use of smaller hand tools, the Dutton assembly provides a more uniform and accurate loading in placing the rotor hub on the rotor shaft than was theretofore known. However, the use of a full circular ring member having an integral lower cone thereon which is adapted to simultaneously contact three different surfaces on the drive shaft and hub presents serious dimensional tolerance problems which in practice would most likely prevent proper loading of the lower cone member on the drive shaft flange, the vertical drive shaft surface and the lower tapered hub surface. If, for example, contact is made between the drive shaft flange and the cone, no pressure can be exerted between the lower tapered hub surface and the cone.
In order to alleviate the problems associated with the Dutton assembly, a bottom circular pressure plate was developed with a separate diametrically split cone. The bottom pressure plate included radial slots which permitted the inner annular edge of the ring to bend upwardly against the cone to load the cone as the plate was bolted to the bottom surface of the hub. This assembly has proven to be not entirely satisfactory since excessive bolt torquing is required to effect bending of the metal circular ring, and proper loading of the split cone is difficult to achieve since the loading forces on the cone are dependent on the toroidal flexibility of the plate and installation tolerances, and elastic deflections of the mechanical parts inhibit perfect allignment of the hub with the split cone.
The present invention provides a helicopter rotor head assembly similar to that disclosed by Dutton in the above-mentioned patent; however, the deficiencies referred to above with respect to the lower cone assembly of Dutton and the slotted pressure plate are eliminated by the use of a lower split cone assembly having an annular plate secured to the bottom of the hub around the rotor shaft and a plurality of loading bolts angularly carried around the plate which are individually torqued to apply upwardly directed forces on the split cone to thereby provide for a balanced loading of the split cone with a minimum of bolt torquing. Sincethe split cone is fabricated separately from the annular plate, machine tolerances required for the instant invention are much lss than that required for the assembly disclosed in the Dutton patent. Also, since the loading forces are directly applied upon the split cone, there is no possibility that a portion of the load forces will be utilized to produce a bending moment in a rigid metal plate such as was the case in the prior art, and the degree of loading can be better controlled. The angular positioning of the bolts serves to reduce friction along the tapered contact surfaces and provides a radial force component to the cone to increase joint efficiency.
It is therefore the main object of the invention to provide an improved helicopter rotor head assembly wherein the rotor hub is secured to the helicopter drive shaft in a simple and economical manner while producing a non-vibrational rotation of the hub on the drive shaft.
It is another main object of the invention to provide an assembly of the character set forth above, wherein a lower split cone is loaded between a drive shaft and a hub by means of individual loading bolts mounted on a separate annular plate affixed to the bottom of the rotor hub.